Appearance of Birbeck granule-like structures in anti-T6 antibody-treated human epidermal Langerhans cells

Birbeck granule-like "organized smooth endoplasmic reticulum" resulting from the expression of a cytoplasmic YFP-tagged langerin

Langerin is required for the biogenesis of Birbeck granules (BGs), the characteristic organelles of Langerhans cells. We previously used a Langerin-YFP fusion protein having a C-terminal luminal YFP tag to dynamically decipher the molecular and cellular processes which accompany the traffic of Langerin. In order to elucidate the interactions of Langerin with its trafficking effectors and their structural impact on the biogenesis of BGs, we generated a YFP-Langerin chimera with an N-terminal, cytosolic YFP tag. This latter fusion protein induced the formation of YFP-positive large puncta. Live cell imaging coupled to a fluorescence recovery after photobleaching approach showed that this coalescence of proteins in newly formed compartments was static. In contrast, the YFP-positive structures present in the pericentriolar region of cells expressing Langerin-YFP chimera, displayed fluorescent recovery characteristics compatible with active membrane exchanges. Using correlative light-electron microscopy we showed that the coalescent structures represented highly organized stacks of membranes with a pentalaminar architecture typical of BGs. Continuities between these organelles and the rough endoplasmic reticulum allowed us to identify the stacks of membranes as a form of “Organized Smooth Endoplasmic Reticulum” (OSER), with distinct molecular and physiological properties. The involvement of homotypic interactions between cytoplasmic YFP molecules was demonstrated using an A206K variant of YFP, which restored most of the Langerin traffic and BG characteristics observed in Langerhans cells. Mutation of the carbohydrate recognition domain also blocked the formation of OSER. Hence, a “double-lock” mechanism governs the behavior of YFP-Langerin, where asymmetric homodimerization of the YFP tag and homotypic interactions between the lectin domains of Langerin molecules participate in its retention and the subsequent formation of BG-like OSER. These observations confirm that BG-like structures appear wherever Langerin accumulates and confirm that membrane trafficking effectors dictate their physiology and, illustrate the importance of molecular interactions in the architecture of intracellular membranes.

Langerin/CD207 sheds light on formation of birbeck granules and their possible function in Langerhans cells

Langerhans cells (LCs) are immature dendritic cells of epidermis and epithelia, playing a sentinel role through their specialized function in antigen capture, and their capacity to migrate to secondary lymphoid tissue to initiate specific immunity. A unique feature of LCs is the presence of Birbeck granules (BGs), which are disks of two limiting membranes, separated by leaflets with periodic "zipperlike" striations. The recent identification of Langerin/CD207 has allowed researchers to decipher the mechanism of BG formation and approach an understanding of their function. Langerin is a type II lectin with mannose specificity expressed by LCs in epidermis and epithelia. Remarkably, transfection of Langerin cDNA into fibroblasts creates a dense network of membrane structures with features typical of BGs. Furthermore, mutated and deleted forms of Langerin have been engineered to map the functional domains essential for BG formation. Langerin is a potent LC-specific regulator of membrane superimposition and zippering, representing a key molecule to trace LCs and to probe BG function.

Reproduction of Langerin/CD207 traffic and Birbeck granule formation in a human cell line model

Birbeck granules (BG) are organelles specific to Langerhans cells (LCs), which form where the C-type lectin Langerin accumulates. Their function remains obscure due to morphologic and dynamic alterations induced by maturation of isolated LC. In this study, we attempted to reconstitute Langerin traffic and BG formation in the endosomal pathway of a human melanoma cell line. In the selected Langerin-transfected cell line, M10-22E, Langerin is distributed between the early recycling endosomal compartment and the plasma membrane, as in LC. Whereas mainly concentrated in membranes related to the Rab11(+) endosomal recycling compartment at the steady state, Langerin also recycles in M10-22E cells and drives BG biogenesis in the endosomal recycling compartment. Interruption of endocytosis or recycling induces redistribution of intracellular Langerin with an associated alteration in BG location and morphology. We have, therefore, generated a stable, Langerin-transfected cell line in which Langerin traffic and distribution and BG morphology replicate that seen in freshly isolated LC. This practical model can now be used to further delineate the nature and function of BG.

Identification and Characterization of an Alternatively Spliced Isoform of Mouse Langerin/CD207

The mouse homologue of human Langerin (CD207), a novel Langerhans cells (LC)-restricted C-type lectin that likely participates in antigen recognition and uptake, has been recently identified. In this study, we isolated the mouse Langerin cDNA from murine fetal skin-derived dendritic cells (FSDDC) by subtractive cloning and rapid amplification of cDNA ends (RACE). An alternatively spliced variant of mouse Langerin that lacked the extracellular neck domain (DeltaE3Langerin) was detected in RNA derived from FSDDC and epidermal LC by RT-PCR. In vitro-generated FSDDC and epidermal LC expressed both full-length and DeltaE3Langerin mRNA, but tissue expression was not restricted to skin. Mouse Langerin protein isoforms were readily detected in fibroblasts transfected with cDNAs encoding epitope-tagged Langerin and DeltaE3Langerin. Recombinant DeltaE3Langerin protein localized with transferrin-containing compartments in transfected fibroblasts. Full-length mouse Langerin-bound mannan, whereas DeltaE3Langerin and soluble bacterial recombinant Langerin protein lacking the neck domain did not. Fibroblasts transfected with mouse Langerin cDNA contained typical Birbeck granules (BG) and cored tubules, whereas DeltaE3Langerin cDNA did not induce BG or cored tubule formation in transfected fibroblasts. Developmentally regulated expression of Langerin isoforms provides a mechanism by which Langerin involvement in antigen uptake and processing could be regulated.

Birbeck granules are subdomains of endosomal recycling compartment in human epidermal Langerhans cells, which form where Langerin accumulates

Birbeck granules are unusual rod-shaped structures specific to epidermal Langerhans cells, whose origin and function remain undetermined. We investigated the intracellular location and fate of Langerin, a protein implicated in Birbeck granule biogenesis, in human epidermal Langerhans cells. In the steady state, Langerin is predominantly found in the endosomal recycling compartment and in Birbeck granules. Langerin internalizes by classical receptor-mediated endocytosis and the first Birbeck granules accessible to endocytosed Langerin are those connected to recycling endosomes in the pericentriolar area, where Langerin accumulates. Drug-induced inhibition of endocytosis results in the appearance of abundant open-ended Birbeck granule-like structures appended to the plasma membrane, whereas inhibition of recycling induces Birbeck granules to merge with a tubular endosomal network. In mature Langerhans cells, Langerin traffic is abolished and the loss of internal Langerin is associated with a concomitant depletion of Birbeck granules. Our results demonstrate an exchange of Langerin between early endosomal compartments and the plasma membrane, with dynamic retention in the endosomal recycling compartment. They show that Birbeck granules are not endocytotic structures, rather they are subdomains of the endosomal recycling compartment that form where Langerin accumulates. Finally, our results implicate ADP-ribosylation factor proteins in Langerin trafficking and the exchange between Birbeck granules and other endosomal membranes.

Contact allergens, but not irritants, alter receptor-mediated endocytosis by human epidermal Langerhans cells

Allergic contact dermatitis is a T-cell-mediated inflammation, induced by contact with sensitizers and occurring through the release of epidermal cytokines and the activation of epidermal Langerhans cells (LCs). The aim of this study was to analyse early events of LC activation induced either by contact allergens or by irritants devoid of any contact allergenic properties. in order to obtain an in vitro method to discriminate between these two groups of molecules. Various contact sensitizers and irritants were studied for their effects on the endocytosis of major histocompatibility complex class II (MHC-II) molecules by freshly-isolated human epidermal LCs. As observed by flow cytometry, a spontaneous decrease in the surface expression of MHC-II (HLA-DR) molecules, linked to spontaneous internalization of the MHC-II molecules by LCs, was obtained by moving freshly-isolated LCs from 4 degrees C to 37 degrees C. Pre-incubation of LCs with either sensitizers or irritants increased the spontaneous internalization of HLA-DR molecules with a similar magnitude, but no clear discrimination between sensitizer and irritant effects was obtained by flow cytometry analysis. In contrast, confocal microscopy enabled discrimination between the effects of sensitizers and irritants: sensitizer-treated samples showed internalized HLA-DR molecules aggregated in large vesicles with very bright fluorescence; irritant-treated samples were not different from untreated controls and showed compact HLA-DR molecules in small vesicles with diffuse fluorescence, and mostly localized in the submembranous zone. Electron microscopy demonstrated that sensitizer-treated LCs internalized HLA-DR molecules preferentially in lysosomes collected near the nucleus, whereas the irritant-treated and non-treated LCs internalized these molecules in the prelysosomes only near the cell membrane. We conclude that contact allergens and irritants induce distinct patterns of HLA-I)R endocytosis, which may be useful for the development of in vitro screening tests.

Nosology and pathology of Langerhans cell histiocytosis

The pathology of LCH is continuing to be defined and diagnostic features clarified as unprecedented research is done. Although the etiology of LCH remains unknown, new information on cytokines, viruses, immunologic dysfunction, cell surface antigen expression, and clonality in the disease are providing essential pieces of the puzzle.

Formation of Birbeck granule-like structures in vascular dendritic cells in human atherosclerotic aorta. Lag-antibody to epidermal Langerhans cells recognizes cells in the aortic wall

It has previously been demonstrated that vascular dendritic cells reside in the arterial intima and are involved in human atherogenesis. During the present ultrastructural examination of aortic atherosclerotic lesions, pentalaminal structures, similar to Birbeck granules which uniquely present in Langerhans cells, were found in the cytoplasm of vascular dendritic cells and the formation of these Birbeck granule-like structures from dense granules was identified. To find out how Birbeck granule-like structures might relate to Birbeck granules of Langerhans cells, we used Lag-antibody which specifically stains Birbeck granules and Birbeck granule-associated structures in Langerhans cells. Lag-positive cells were found in the aortic wall. Our observations suggest a close relationship between vascular dendritic cells and Langerhans cells and this may imply that mechanisms of antigen presentation known for Langerhans cells might be similar to those involved in atherosclerosis.

Alterations of dendritic cells in the rat thymus without epithelial cell loss during cyclosporine treatment and recovery

After cyclosporine treatment, dendritic cells disappear from the rat thymic medulla. The present study was undertaken to examine the ultrastructural alterations in the dendritic cell population during 14-day cyclosporine treatment and subsequent 6-week recovery. Four dendritic cell subtypes were defined ultrastructurally by a newly developed classification system. In addition, the potential effect of cyclosporine on six medullary epithelial cell subtypes was studied. During cyclosporine treatment, a prominent reduction of dendritic cells was seen at the ultrastructural level, whereas the total number of medullary epithelial cells remained largely unchanged. These findings were confirmed by immunohistochemistry. The number of mature dendritic cells declined later than the number of immature ones. A decrease in the antigen-processing capacity of remaining dendritic cells was suggested by the disappearance of Birbeck granules and the reduced number of tubulovesicular complexes. These findings support a disturbance of clonal deletion during cyclosporine treatment. The dendritic cell alterations appeared reversible 4 weeks after the restoration of the original architecture. During recovery, dendritic cells displaying lysosomal elements outnumbered those found in the normal uninvoluted thymus. This phenomenon probably reflects an enhanced turnover of cell organelles. No treatment-related effect on epithelial cell subtypes was seen.

The role of dendritic cells in cutaneous immunity

This article reviews the role of dendritic cells in cutaneous immunity. Langerhans cells (LC) found in the epidermis are the best-characterized dendritic cell population. They have the ability to process antigen in the periphery, transport it to the draining lymph nodes (DLN) where they are able to cluster with, and activate, antigen-specific naive T cells. During migration LC undergo phenotypic and functional changes which enable them to perform this function. There are other less well-characterized dendritic cells including dendritic epidermal T cells, dermal dendrocytes and dermal "LC-like' cells. Although there is no evidence that dendritic epidermal T cells (DETC) can present antigen or migrate to lymph nodes, they do influence the intensity of cutaneous immune responses to chemical haptens. Antigen-presenting cells (APC) in the dermis may provide alternative routes of antigen presentation which could be important in the regulation of skin immune responses. Therefore, dendritic cells are vital for the induction of immune responses to antigens encountered via the skin. LC are particularly important in primary immune responses due to their ability to activate naive T cells. The faster kinetics of secondary responses, and the ability of nonprofessional APC to induce effector function in previously activated cells, suggest that antigen presentation in the DLN may be less important in responses to previously encountered antigens. In these secondary responses, dendritic and nondendritic APC in the skin may directly induce effector functions from antigen-specific recirculating cells.

Epidermal Langerhans cells in the terrestrial turtle, Kinosternum integrum

In mammalian epidermis, Langerhans cells (LC) are the only antigen-presenting dendritic cells that possess the ectoenzyme adenosine triphosphase (ATPase) and constitutively express class II molecules encoded by the major histocompatibility complex. Recently, we demonstrated the presence of LC in chicken epidermis. The aim of the present study is to demonstrate the presence of LC-like cells in turtle Kinosternum integrum, epidermis by light and ultrastructural ATPase histochemistry. ATPase-positive dendritic cells were observed in epidermal sheets whose maximum mean number was 192 cells/mm2. Electron microscopy for ATPase stained sections showed an electrondense precipitate in the plasma membrane of dendritic clear cells located among basal and suprabasal keratinocytes, ultrastructurally similar to LC. In serial sections, some dendritic cells showed LC (Birbeck) granules. The present study demonstrates for the first time ATPase-positive dendritic cells, morphologically similar to LC, in reptilian epidermis.

Antigen processing: the gateway to the immune response

The T-lymphocyte response to an antigen is governed by the source of that antigen and the way in which it is processed. Before recognition by T lymphocytes, proteins must be degraded to peptides by antigen-presenting cells. The peptides are then presented on major histocompatibility complex (MHC) molecules for recognition by the T cells. Antigens arising outside the cell (e.g., bacteria) are phagocytosed and processed by the exogenous pathway for presentation on MHC class II molecules (e.g., DR) to CD4+ cells. Antigens derived from the cytoplasm (e.g., viral proteins) are processed by the endogenous pathway for presentation by MHC class I molecules (e.g., HLA-A, -B, -C) to CD8+ cells. The response to a hapten or drug is a function of the antigen processing pathway and is determined by its chemical properties. Antigen processing also governs the T-cell response to pathogens, vaccines, and autoimmune conditions.

Effects of immunological responsiveness on Langerhans cell behavior in contact sensitization

The epicutaneous application of haptens results in a functional activation of the antigen-presenting Langerhans cells (LCs) which is necessary for the induction of contact sensitivity. In this ultrastructural study, we investigated the effects of the immune response on these cellular properties of the LCs by using 2 strains of guinea pigs with genetically determined high and non responsiveness, respectively, to the strong sensitizer 2,4-dinitrochlorobenzene (DNCB). After skin painting, both strains showed a similar cellular and endocytotic activation of the LCs and a similar intraepidermal localization of DNCB on immunoelectron microscopical visualization. There were however few LC-lymphoid cell interactions in the non responders, in contrast to numerous such appositions in the other strain. Intravenous tolerization with 2,4-dinitrobenzene-1-sulfonic acid, which is known to block the DNCB receptor of T cells, hampered the lymphoid cell interactions in the DNCB treated high responders, but it did not affect the LC activation. These in vivo observations demonstrate that the hapten-induced changes of the LC properties is the initial, T-cell independent event in contact sensitization.

Internalization of surface HLA-DR molecules by human epidermal Langerhans cells: analysis by flow cytometry and confocal microscopy

Langerhans cells (LC) play a pivotal role in antigen processing and presentation to T cells during delayed-type hypersensitivity reaction in the skin. Antigen presentation involves the interaction between the class II molecules of MHC (HLA-DR) expressed by LC and T receptor of CD4+ T lymphocytes. It is now recognized that class II molecules are internalized into LC and can be associated with processed immunogenic peptides. This process involves receptor-mediated endocytosis. The aim of this study was to investigate the time-course of endocytosis of HLA-DR by freshly isolated human LC. Epidermal cells, obtained from normal skin samples, were labeled by indirect immunofluorescence using anti-HLA-DR monoclonal antibodies (MAb). The cell suspension was incubated at 37 degrees C for different periods (15, 30, 45, 60 and 90 min) and then analyzed by flow cytometry and confocal microscopy. Flow cytometry analysis showed decreased HLA-DR molecule expression by LC after incubation at 37 degrees C. Confocal microscopic analysis showed different strain patterns depending on the incubation time: (1) T = 0, continuous peripheral staining; (2) T = 15 min, patchy peripheral staining; (3) T = 30 min, patches or intracellular vesicular staining; (4) T = 45 min, intracellular vesicular staining; (5) T = 60 min, diffuse intracellular staining; (6) T = 90 min, aggregated staining. In our study model, flow cytometry provides quantitative information for the HLA-DR endocytosis, whereas confocal microscopy provides qualitative results concerning the intracellular distribution of internalized HLA-DR molecules.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraepithelial dendritic cells and selective activation of Th2-like lymphocytes in patients with atopic asthma

This study examines the distribution of intraepithelial dendritic cells in eight atopic patients with symptomatic asthma and their ability to induce activation of autologous T lymphocytes in vitro. All subjects were sensitized to Dermatophagoides pteronyssinus. The incubation of asthmatic epithelial cells and dendritic cells with autologous resting CD4-positive T cells and purified extracts of D pteronyssinus induced T cell activation and release of high levels of interleukin-4 (IL-4) and interleukin-5 (IL-5). The antigen-presenting activity of dendritic cells was potentiated by epithelial cell-derived granulocyte-macrophage colony-stimulating factor (GM-CSF), since an antibody against GM-CSF reduced it. Circulating monocytes of the two groups of donors were equally effective in promoting selective activation of IL-4- and IL-5-producing T cells. Thus, an interaction between dendritic cells and allergens may favor local activation of CD4-positive T cells with Th2-like function in atopic asthmatic subjects, thereby promoting the expression of the disease.

Three-dimensional reconstruction and stereometric analysis of Langerhans cells in mouse epidermis

In the presented studies stereometric analysis and spatial reconstruction was performed on two Langerhans cell (LC) types. One was free of LC-I and the other contained LC-II Birbeck granules in the perinuclear space. The presented stereometric analysis demonstrated significant differences between the so-distinguished two cell types. Differences were observed not only in the number and distribution of Birbeck's granules but also in the areas of smooth and rough endoplasmic reticulum, in the area of vesicles surrounding Golgi apparatus, in the volume of cisterns of the apparatus, and in the ratio of cell nucleus area to its volume. Differences noted between the two cell types were of quantitative character. They might result from different stages of differentiation of the cells from their precursors in the epidermis or from distinct functional stages of the cells.

Alpha IIB beta 3 integrin dissociation induced by EDTA results in formation of Birbeck granule-like structures in human blood platelets

EDTA (5 mM at 37 degrees C and pH 7.4 for 30 min) induces morphologic changes in the platelet surface-connected canalicular system (SCCS), in particular the collapse of some portions of the canaliculi. These collapsed elements are made up of two parallel limiting membranes and a central irregular striated zone, a pentalaminar organization that clearly resembles that of Langerhans cell Birbeck granules (BG). Such BG-like structures are also seen between adjacent platelets in EDTA-treated platelet microaggregates. EDTA is known to induce an irreversible dissociation of the platelet membrane glycoprotein(GP)IIb-IIIa, the alpha IIb beta 3 platelet-specific intergrin, a calcium-dependent heterodimer that serves as an inducible receptor for fibrinogen and is essential for platelet aggregation. Hence, we looked for involvement of the GPIIb-IIIa in the formation of these BG-like modifications. We observed that these changes i) cannot be induced in type I Glanzmann's thrombasthenia, where the GPIIb-IIIa complexes are absent; ii) did not appear when human platelets were pre-incubated with MoAb anti-GPIIb-IIIa complex, which protected GPIIb-IIIa from EDTA-induced dissociation; iii) appeared only at alkaline pH and 37 degrees C, which corresponds to the range of pH and temperature where EDTA can dissociate the GPIIb-IIIa complexes; iv) are accompanied by the dissappearance on fluorescence flow cytometry analysis of the heterodimer specific epitopes; and v) do not appear in rat platelets at pH 7.4 where GPIIb-IIIa does not dissociate after EDTA treatment. Thus, the appearance of BG-like structures in the platelet SCCS is directly dependent on the EDTA-induced dissociation of the GPIIb-IIIa complexes. Furthermore, using gold-labeled MoAb concomitantly with the addition of EDTA, we observed that only GPIIb is present in the collapsed portions of the canaliculi. On Lowicryl thin sections essentially polyclonal antibodies to GPIIb labeled the central striated zone. All these observations lead us to suggest that homopolymers of GPIIb could be responsible for "zipping" of the SCCS and raise the question of the participation of a Langerhans cell integrin in the formation of Langerhans cell BG.

Internalization of Ia molecules into Birbeck granule-like structures in murine dendritic cells

Dendritic cells isolated from the draining lymph nodes of mice sensitized epicutaneously with hapten are potent antigen-presenting cells and contain Birbeck granules and cored tubules characteristic of antigen-activated epidermal Langerhans cells. We used immunogold labeling and transmission electron microscopy to follow the internalization of Ia molecules in these antigen-presenting cells. We found that Ia molecules were internalized into Birbeck granule-like structures in the antigen-activated dendritic cells. Computer reconstruction of serial sections of the dendritic cells demonstrated that these structures span the cytoplasm from the cell membrane to the nuclear membrane and are associated with lysosomes. The internalization of Ia molecules into these structures supports the hypothesis that the Birbeck granule-like structures are derived from the cell membrane and are involved in the antigen-processing/presenting function of the dendritic cells.

Cytomembrane-derived Birbeck granules transport horseradish peroxidase to the endosomal compartment in the human Langerhans cells

It has been previously shown that the cytomembrane of human Langerhans cells (LC) has the capacity to fold upon itself, thereby forming Birbeck granules (BG), which then internalize. I confirmed this by exposing LC in vitro to horseradish peroxidase (HRP) at +8 degrees C and +37 degrees C. On incubation at +37 degrees C the label appeared not only in BG, but also in tubular structures and vesicles of different shapes and sizes. Interconnections between these labeled endosomal structures were common. The LC cytomembrane could form BG at +8 degrees C and, moreover, the BG was the only organelle that was labeled and internalized at this temperature. Thus, cytomembrane-derived BG are endocytotic in nature and link the exterior of the cell and the endosomal compartment. The membrane interlinking of the BG may eventually dissolve and the BG then transform into an endosomal vesicle.

Birbeck granules or Birbeck junctions? Intercellular "zipperlike" lattice junctions in eosinophilic granuloma of bone

Three cases of eosinophilic granuloma of bone exhibited intercellular attachments between histiocytes, with a pentalaminar structure identical to that seen in nearby intracellular Birbeck granules (BG). It is proposed that the term "lattice junction" be coined to describe this organelle, which seems to be expressed only by cells of monocytic-histiocytic lineage. While this finding confirms the ability of the surface membrane to form such structures, it does not necessarily preclude other intracytoplasmic sites of origin for some BG. The previously suggested role of BG in membrane storage and regulation is reiterated as a supportable model for their function.

Ultrastructural similarities between epidermal Langerhans cell Birbeck granules and the surface-connected canalicular system of EDTA-treated human blood platelets

Birbeck granules characterize under the electron microscope epidermal Langerhans cells. These distinctive pentalaminar organelles are indeed not detectable in the possible precursors of human Langerhans cells and tend to disappear in cultured human Langerhans cells. The mechanisms that lead to the appearance of Birbeck granules in epidermal Langerhans cells and to their later disappearance still remain unknown. In the present study we show that the more or less dilated elements of the surface-connected canalicular system of human blood platelets collapse after EDTA treatment. Made up of two parallel limiting membrane and central irregular striated density, these elements show great ultrastructural similarities with the Birbeck granules of human epidermal Langerhans cells. These platelet morphologic changes i) are directly dependent on the EDTA-induced dissociation of the glycoprotein GP IIb-IIIa, the platelet-specific calcium-dependent heterodimer complex, member of the beta 3 integrin subfamily (alpha IIb beta 3) and ii) apparently result from a cross-linking of the dissociated glycoproteins. These findings lead us to propose that in the same manner cells of the Langerhans lineage, on reaching the epidermis, will find themselves in contact with an epidermal specific ligand. Interactions between this epidermal ligand and Langerhans cell receptors could then induce, all along the circuit taken by the ligand-receptor complexes, morphologic modifications, i.e., appearance of structures of Birbeck granule type.

The Langerhans cell: an underestimated cell in atopic disease

Langerhans cells (LC) are very potent antigen-presenting cells. In atopic disorders such as allergic rhinitis and atopic dermatitis LC are known to bear IgE surface molecules. IgE-positive LC can bind allergen and present it to T lymphocytes to induce an allergen-specific T-cell response and IgE synthesis. Therefore, IgE-bearing LC might play an important role in the triggering of the immune system to maintain ongoing IgE synthesis. The importance of the IgE-bearing LC in atopy has not been assessed but deserves further investigation to find out more about the part played by these cells, not only in the atopic disorders described here but also in others such as gastrointestinal allergy and allergic asthma.

ATPase and morphologic changes in Langerhans cells induced by epicutaneous application of a sensitizing dose of DNFB

We have previously described an ATPase Langerhans cell (LC) staining technique allowing progression from light to electron microscope observation. Using this technique we have studied, following epicutaneous application of a sensitizing dose of a hapten, 2,4-dinitro-1-fluorobenzene (DNFB), the fate of the epidermal LC located in the sensitization zone. We wanted to know, under the light microscope, if the density and/or morphology of the LC are modified by such a treatment and, under the electron microscope, what are the ultrastructural changes accompanying the possible light microscope modifications. Under the light microscope, the observation of LC during the 5 d necessary for the development of contact sensitivity to DNFB shows that their number drops in the course of the first 24 h to normalize again 3 d later. Under the electron microscope, observations over the first 24 h revealed that LC remained in the epidermis, but were ATPase-negative. The disappearance of the membrane ATPase activity took place while the LC presented an increased number of coated pits, coated vesicles, endosomes, and lysosome organelles which characterize, at the ultrastructural level, the process of receptor-mediated endocytosis (RME). Following RME, many Birbeck granules (BG) appeared in the cytoplasm. Thus, epicutaneous application of DNFB leads to an endocytic activation of LC. However, the ligand(s) and/or the cell-surface components, which probably internalize during the RME process, remain unknown.